An apparatus for transferring a wafer comprising displaceable transferring means for engaging the wafer and means for determining a horizontal position of the wafer relative to said displaceable transferring means is described by U.S. Pat. No. 5,768,125, hereafter referred to as Zinger et al. and assigned to applicant, the subject matter of which is incorporated herein by reference in its entirety. Said apparatus comprises an array of light sensors positioned in the path of the wafer in a direction not parallel with the extension of said path. The light sensors detect the passage of the wafer edge. Calculation means are provided for determining the position of said article based on the output of said array of sensors.
The apparatus is capable of measuring the position of one wafer at a time. However, when loading a large batch of wafers in a wafer boat, the loading process takes a significant amount of time. It would be desirable to be able to load two or more wafers simultaneously in one loading movement of the wafer transfer robot without sacrificing the capability of measuring the position of each wafer being transferred. It is the object of the present invention to provide a solution for this problem and to provide an apparatus for transferring wafers comprising displaceable transferring means capable of transferring two or more wafers simultaneously while measuring the positions of the two or more wafers relative to the displaceable transferring means.